1. Field of the Invention
The present invention relates to a dielectric lens antenna and a radio device including the same, and more particularly to a dielectric lens antenna for use in a radio device operable in a microwave band and a millimetric wave band such as a radar for preventing motorcar collisions, and a radio device including the same.
2. Description of the Related Art
In radio devices such as radars for preventing motorcar collisions and so forth, dielectric lens antennas are used as a means for controlling the directivity of radio waves. FIG. 6 is a cross section of a conventional dielectric lens antenna. The dielectric lens antenna as shown in FIG. 6 is disclosed in detail in Japanese Unexamined Patent Publication No. 6-6128.
In FIG. 6, the dielectric lens antenna 1 comprises a dielectric lens 2 having a substantially disk shape, a primary radiator 3, and a dielectric member 4 having a lower dielectric constant than the dielectric lens 2, provided between the dielectric lens 2 and the primary radiator 3. The primary radiator 3 is disposed at the back focal point of the dielectric lens 2. The dielectric member 4 is formed in a substantially circular cone shape in which the primary radiator 3 is positioned at the apex, and the dielectric lens 2 is provided at the base, and its dielectric constant is uniform. Further, the dielectric lens 2 and the primary radiator 3 are connected through and secured to the dielectric member 4.
In the dielectric lens antenna 1 configured as described above, the thickness of the dielectric lens 2 can be reduced, and moreover, it is unnecessary to provide a holder for holding the dielectric lens 2 at a predetermined position with respect to the primary radiator 3.
For reduction of the thickness of such a dielectric lens antenna, there are proposed methods of increasing the dielectric constant of a dielectric lens in order to make the dielectric lens thinner, shortening the back focal distance of the dielectric lens so that the distance between the dielectric lens and the primary radiator is reduced, or increasing the dielectric constant of a dielectric member so that the distance between the primary radiator and the dielectric lens is reduced, and so forth.
However, there is the problem that when the dielectric constant of a dielectric lens is increased, the efficiency of the dielectric lens itself is reduced.
Further, to reduce the back focal distance of the dielectric lens, it is necessary to increase the thickness of the dielectric lens, and as a whole, the thickness of the dielectric lens antenna can not be reduced. Further, this causes the problem that the efficiency deteriorates. Further, since materials with which dielectric lenses are formed have a high heat shrinkage, dielectric lenses which are thick can not be injection-molded with high dimensional precision.
In the methods for increasing the dielectric constant of the dielectric member, phase-shifting increases, due to the routes of radio waves between the primary radiator and the dielectric lens. Accordingly, there is the problem that the dielectric lens antenna can not operate normally.
Accordingly, it is an object of the present invention to solve the above-described problems, and provide a dielectric lens antenna which can be thinner and having a high efficiency.
It is also an object of the invention to provide a radio device including such a dielectric lens antenna.
To achieve the above object, according to the present invention, there is provided a dielectric lens antenna which comprises a dielectric lens, a primary radiator, and a dielectric member provided between the dielectric lens and the primary radiator, the dielectric member having a dielectric constant distributed nonuniformly therein.
Preferably, in the dielectric lens antenna of the present invention, the dielectric member is formed into a substantially circular cone shape in which the dielectric lens is positioned on the base, and the primary radiator is provided at the apex, and the dielectric constant is reduced in the radial direction of the dielectric lens from a center line thereof passing through the center of the dielectric lens and the primary radiator.
Preferably, in the dielectric lens antenna of the present invention, the dielectric member has such a configuration that the dielectric constant is reduced continuously in the radial direction of the dielectric lens in conformity to a substantially circular cone pattern.
In the dielectric lens antenna of the present invention, the dielectric member is preferably formed of plural layers each having a substantially circular cone shape so that the dielectric constant is reduced stepwise in the radial direction of the dielectric lens.
Preferably, in the dielectric lens antenna of the present invention, the thickness of the largest area portion in each layer having an even dielectric constant in the dielectric member is up to the effective wavelength of the radio wave with a used frequency in the layer.
A radio device according to the present invention includes any one of the above-described dielectric lens antennas.
With the configuration as described above, the dielectric lens antenna of the present invention can be rendered of high efficiency and can be made thinner.
The radio device of the present invention can be miniaturized, due to the thinning of the dielectric lens antenna.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.